1. Field of the Invention
The present invention relates to a substrate cleaning method and a substrate cleaning apparatus for cleaning a substrate, which use, as a cleaning liquid, an aqueous solution of ammonium fluoride or a mixture of an aqueous solution of ammonium fluoride and hydrofluoric acid. More particularly, the present invention is concerned with a novel substrate cleaning method and a novel substrate cleaning apparatus, which are developed for achieving both uniform and stable cleaning treatment and reduction in the amount of the cleaning liquid used.
2. Description of the Related Art
An aqueous solution of ammonium fluoride (including arbitrary mixtures of hydrofluoric acid and ammonia, and ones containing a surfactant) is widely used in the form of a mixture thereof with hydrofluoric acid and the like for cleaning or etching a semiconductor (mainly a silicon oxide film) substrate or a glass substrate in the production process for semiconductor or liquid crystal display (LCD).
In addition to the above example of substrate cleaning, in Unexamined Japanese Patent Application Laid-Open Specification No. 2000-150447, a mixture of ammonia and hydrogen peroxide is used for cleaning silicon wafers. This prior art document discloses that the ammonia concentration and hydrogen peroxide concentration of the mixture are individually monitored during the cleaning, and the change with time of the chemical liquid concentration is accurately estimated based on the results of measurements to calculate and adjust the amount of the chemical liquid to be added, so that the chemical liquid concentration is kept constant.
In these fields, for achieving reduction in weight and downsizing of products and the lowering of consumed electric power, development of a microfabrication technology for a higher integration degree is desired.
In any case of cleaning substrates using, as a cleaning liquid, for example, an aqueous solution of ammonium fluoride, a mixture of an aqueous solution of ammonium fluoride and hydrofluoric acid as mentioned above, or other chemical liquids, a higher accuracy treatment is desired.
By the way, in the above-mentioned cleaning or etching, the chemical component content, such as ammonium fluoride (NH4F), and hydrogen fluoride (HF), and moisture content of the cleaning liquid change (evaporate) with the lapse of time. Therefore, such cleaning or etching has a problem in that the etching rate for a silicon oxide film or a glass substrate changes (increases).
This is caused by the gradual increase in the hydrofluoric acid component concentration of the cleaning liquid with the lapse of time during the use of the cleaning liquid, and the problem of the change in the concentrations of the components in the cleaning liquid has been conventionally solved by frequently replacing the cleaning liquid.
However, in this method, a great amount of the cleaning liquid is required. Especially a cleaning liquid containing ammonium fluoride is used in an ammonium fluoride concentration as high as several tens % (for example, about 40% by weight), and hence, the amount of the chemical consumed per liquid replacement in this cleaning liquid is large, as compared to those in other cleaning liquids generally used in a chemical concentration of about several %. Accordingly, the amounts of the ammonium fluoride and hydrofluoric acid used (cost for chemicals) are to increase.
Further, after the cleaning liquid is used, a waste water treatment shown in FIG. 1 is needed, and, in this treatment, a large amount of resource (waste water treatment agent) is consumed, resulting in a large amount of waste products (waste water and sludge)(see FIG. 2).
FIG. 1 is an explanatory view illustrating a process for treating the effluent of a cleaning liquid. Effluent produced in a cleaning bath 11 is transferred to a pH-adjusting bath 12, and neutralized in this bath by, for example, a 20% aqueous solution of calcium hydroxide. Next, the effluent is transferred to a coagulo-sedimentation bath 13 and subjected to coagulation and sedimentation by a chemical, such as aluminum sulfate, and then, in a flocculation bath 14, is flocculated by a polymeric flocculent, such as a polyacrylamide flocculent, and subjected to sedimentation, followed by discharge in the form of sewage and sludge.
In the above treatment process, as shown in FIG. 2, for treating 1 kg of a 40% aqueous solution of ammonium fluoride, 2.0 kg of 20% calcium hydroxide, 0.3 kg of 8% aluminum sulfate, and 1.6 kg of a polymeric flocculant are required, so that 2.6 kg of sludge having a solids content of 70% and 2.3 kg of sewage waste water are produced.
In recent years, concern about the environmental problems is growing worldwide, and a demand for removing the environmental burden caused by the production of substrates for semiconductor and LCD is being increasing. Therefore, with respect to the cleaning liquid for the substrates, not only for reducing the cost, but also for achieving preservation of the environment, i.e., a saving of resource, a reduction in waste, and environmental cleaning, a solution of the problem about the reduction in the amount of the cleaning liquid used is socially desired strongly.